Radiation astronomy/Greens

Northern Lights are usually green, but in this image there is the very rare blue light. Credit: Varjisakka.

Green astronomy is the study and use of emission and absorption lines or bands in the wavelength range 495-570 nm. The use of filters with respect to this wavelength range are common when studying the Sun. Green astronomy also studies green sources and objects.

Green, blue and red are additive colors. All the colors you see on your computer screen are made by mixing them in different intensities. Credit: Bb3cxv.

The word green has the same Germanic root as the words for grass and grow and is a common color reflected by leaves on Earth. Credit: The cat.

Malachite is a mineral occurring on Earth, like many greens, is colored by the presence of copper, specifically by basic copper(II) carbonate.[1] Credit: Rob Lavinsky.

Green has a wavelength range of approximately 520–570 nm,[2] a frequency range of ~575–525 THz,[2] with color coordinates of (0, 255, 0) and a hexagonal triplet of #00FF00 from sRGB source of sRGB approximation to NCS S 2060-G.[3]

Def. "[t]he colour of growing foliage, as well as other plantcells containing chlorophyll; the colour between yellow and blue in the visible spectrum; one of the primary additive colour for transmitted light; the colour obtained by subtracting red and blue from white light using cyan and yellow filters"[4] is called green.

"...in nature chiefly conspicuous as the colour of growing herbage and leaves..."[5]

"The perception of greenness (in opposition to redness forming one of the opponent mechanisms in human color vision) is evoked by light which triggers the medium-wavelength Mcone cells in the eye more than the long-wavelength L cones. Light which triggers this greenness response more than the yellowness or blueness of the other color opponent mechanism is called green. A green light source typically has a spectral power distribution dominated by energy with a wavelength of roughly 487–570 nm. More specifically, "blue green" 487–493 nm, "bluish green" 493–498 nm, "green" 498–530 nm, "yellowish green" 530–559 nm, "yellow green" 559–570 nm.[8]"[2]

"Green earth is a natural pigment ... It s composed of clay colored by iron oxide, magnesium, aluminum silicate, or potassium. Large deposits were found in the South of France near Nice, and in Italy around Verona, on Cyprus, and in Bohemia. The clay was crushed, washed to remove impurities, then powdered. It was sometimes called Green of Verona.[9]"[2]

This is a detail of a frieze on a wall of tomb QV66, the burial place of Nefertari (c. 1295-1255 B.C.), royal wife of Ramesses the Great, featuring the Egyptian god Osiris. Credit: Mrgoodgame.

This is an image of Rahu the head of the demonic snake. Credit: E. A. Rodrigues.

When any effort to acquire a system of laws or knowledge focusing on an astr, aster, or astro, that is, any natural body in the sky especially at night,[10] discovers an entity emitting, reflecting, or fluorescing green, succeeds even in its smallest measurement, green astronomy is the name of the effort and the result. Once an entity, source, or object has been detected as emitting, reflecting, or fluorescing green, it may be necessary to determine what the mechanism is. Usually this information provides understanding of the same entity, source, or object. Green is a color that suggests the sense of sight. Most people associate astronomy with the sense of seeing, what can be termed visual astronomy. As telescope optics transmit green well, green astronomy is also a field within optical astronomy.

Does the entity transit, continue occupation of a position for longer periods, or apparently change course or position in the sky? Is it a wanderer?

Are biological life forms able to perform astronomy in the green portion of the visible spectrum? If so the organisms may be conducting astrobiology.

Have observers recorded images of sky entities in the green?

At left is Rahu. "Rahu ... is God of the Ascending / North lunar node. Rahu is the head of the demonic snake that swallows the sun or the moon causing eclipses, according to Hindu scriptures. He is depicted in art as a dragon with no body riding a chariot drawn by eight black horses. He is a TamasAsura who does his best to plunge any area of one's life he controls into chaos."[11]

Of the planets in orbit around the Sun, only the Earth occasionally is seen to have green over large areas of its surface. This is usually from the presence of chlorophyll which obtains its green color from magnesium. The first image at right has green from this organic pigment.

Both the image at the page top and the second one at right show green colors in or above the atmosphere of the Earth.

"The Earth of the early Archean (3,800 to 2,500 million years ago) may have had a different tectonic style. During this time, the Earth's crust cooled enough that rocks and continental plates began to form. Some scientists think because the Earth was hotter, that plate tectonic activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material. This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Others argue that the subcontinental lithospheric mantle is too buoyant to subduct and that the lack of Archean rocks is a function of erosion and subsequent tectonic events."[12]

Chlorophyll is a common source of green on Earth's surface. Malachite is a mineral that occurs in rocks at or near the interface between Earth's atmosphere and crust.

"[T]he typical moonless night-sky optical spectrum for the [Calar Alto observatory] ... shows a strong contamination by different [light] pollution lines, in particular from Mercury lines ... Light pollution arises principally from [tropospheric] scattering of light emitted by sodium and mercury vapour and incandescent street lamps ... the mercury lamps produce [a] narrow [line] at 5461 Å ... the 5460Å Hg line lies in the centre of the y-band of the ubvy Strömgren photometric system, which may affect the programs devoted to the study of stellar populations that use this filter set."[15]

"[E]xposures were taken through the F492M filter (4906 Å/364 Å) to cover the [O III] λλ4959, 5007 emission. A single exposure was taken through each of the filters F547M (5454 Å/438 Å) and F718M (7160 Å/595 Å) to record the off-band continua."[16]

The spectrum shows the lines in the visible due to emission from elemental helium. Credit:Teravolt.

"C. FRIEDLÄNDER and H. Kayser have independently claimed to have found helium in the [Earth's] atmosphere. On examination of some photographs of the spectrum of neon I have identified six of the principal lines of helium, which thus establishes beyond question the presence of this gas in the air. The amount present in the neon it is, of course, impossible to estimate, but the green line (wave-length 5016 [501.6 nm]) is the brightest, as would be expected from the low pressure of the helium in the neon."[17]

Helium has green lines at 501 and 505 nm, while a line at 493 nm is in the cyan and 587 nm is in the yellow.[18]

Oxygen has an emission line that occurs in plasmas at 527.62 nm from O IV.[19]

"Since the early work of Ångström,* we have the published records of over a hundred investigations on the spectrum, and many others on the origin or other phenomena characteristic of the aurora."[23] "Babcock, using a Fabry and Perot interferometer, determined very accurately the wave-length of the auroral green line 5577. ... After a careful examination of all the results obtained in these reports, we may only say that the exact nature of the cosmical rays, responsible for the aurora, remains a mystery. ... The origin of the most prominent and interesting line of the auroral spectrum, the line 5577, has hitherto remained unexplained. Vegard* has recently obtained a luminescent band from solid nitrogen, that he supposes, under very special conditions, may coincide with the auroral green line. ... spectra of pure helium and of pure oxygen were taken at different pressures and with various excitations, but no trace of 5577 or of any other new lines was obtained. ... Mixtures of helium, oxygen and nitrogen were excited, and it was found that the line 5577 could be photographed on the same plate with the nitrogen band system, thus reproducing in the laboratory practically the entire auroral spectrum. In ... mixtures of neon and oxygen ... neon enhanced the line 5577 in the same manner as helium. ... From Plate 20 it will be seen that all the lines except 5577 have been identified as strong lines in the spectrum of helium, hydrogen, oxygen, or mercury. ... It has been shown that this line must be attributed to some hitherto unknown spectrum of oxygen, and that it is not a limiting member of the ordinary band spectrum of oxygen. It has been observed faintly in highly purified oxygen when currents of high density have been used."[23]

This is a small volcanic bomb of (black) basanite with (green) dunite. Credit: B.navez.

At right is a visual close up of green sand which is actually olivine crystals that have been eroded from lava rocks. Some olivine crystals are still inside the lava rock.

"Forsterite (Mg2SiO4) is the magnesium rich end-member of the olivine solid solution series."[24]

"Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites. In 2005 it was also found in cometary dust returned by the Stardust probe.[25] In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.[26]"[24]

"Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric). Both are mainly known from meteorites."[24]

At lower right is an image of a small volcanic bomb of (black) basanite with (green) dunite.

"Dunite ... is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group of mantle-derived rocks. Dunite and other peridotite rocks are considered the major constituents of the Earth's mantle above a depth of about 400 kilometers. Dunite is rarely found within continental rocks, but where it is found, it typically occurs at the base of ophiolite sequences where slabs of mantle rock from a subduction zone have been thrust onto continental crust by obduction during continental or island arc collisions (orogeny). It is also found in alpine peridotite massifs that represent slivers of sub-continental mantle exposed during collisional orogeny. Dunite typically undergoes retrograde metamorphism in near-surface environments and is altered to serpentinite and soapstone."[27]

Oxidation states of vanadium are shown from left +2 (lilac), +3 (green), +4 (blue) and +5 (yellow). Credit: Steffen Kristensen.

"The chemistry of vanadium is noteworthy for the accessibility of the four adjacent oxidation states 2-5. In aqueous solution the colours are lilac V2+(aq), green V3+(aq), blue VO2+(aq) and, at high pH, yellow VO42-."[31]

"Carroll and McCormack (1972) in Dublin reported complex spectra in the blue and green wavelength regions of both FeH and FeD".[34]

"Carroll et al. (1976) detected a number of coincidences between laboratory lines of FeH and weak unidentified solar lines, again in the blue and green wavelength region, in addition to the infrared."[35]

The green rim and flashes of a setting Sun are imaged. Credit: Brocken Inaglory.

"As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.[38] The lower rim of an astronomical object is always red.
A green rim is very thin, and is difficult or impossible to see with the naked eye. In usual conditions, a green rim of an astronomical object gets fainter, when an astronomical object is very low above the horizon because of atmospheric reddening,[39] but sometimes the conditions are right to see a green rim just above the horizon."[40]

A number of emission lines have been detected in solar limb faculae.[30]

Def. "the radial distance q from the Sun's center such that the following finite Fourier transform is zero:

where s is a dummy variable, G is the observed solar intensity as a function of the radius, and the parameter a determines the extent of the solar limb used"[42] is called the solar edge.

"When F(G; q, a) = 0, the a dependence of q can be used to choose different points as the edge."[42]

"The wavelength dependence of the facular excess brightness ... is ... normalized to unity for the [intensity of transmission of] the green filter [(525±70 nm)]. ... An unweighted least-squares fit to the five points [(five filter transmission intensities)] gives

"[T]he blue contrast lies about 1 σ above the λ-1 curve. ... [I]f real, [this] may be the result of an increasing opacity in the blue and an increasing ΔT/T in the upper layers of the photosphere. An increasing opacity in the blue may be due to line haze since the blue filter has a 78 nm width."[41]

A picture of the solar corona taken with the LASCO C1 coronagraph. The image is color coded for the doppler shift of the FeXIV 530.8 nm line. Credit: NASA and NRL.

In the image at right the iron (Fe XIV) green line is followed by doppler imaging to show associated relative coronal plasma velocity towards (-7 km/s side) and away from (+7 km/s side) the large angle spectrometric coronagraph LASCO satellite camera.

"The striking absence of green emission above both polar regions at activity minimum led Waldmeier (1957) to use the German term 'Koronalöcher', ie, coronal holes."[43] "Here we restrict ourselves to a qualitative study of large scale structures of the green emission line corona."[43]

The image of the solar coronal cloud at above right shows the polar coronal holes.

"The course of the solar cycle can ... be tracked by changes in the line blanketing. The line-blanketing variations during solar cycle 21 have been measured by Mitchell & Livingston (1991) who found that in disk-integrated spectra the spectral lines in the 500 to 560 nm range are on average 1.4% shallower and have 0.8% smaller equivalent widths at solar maximum than at solar minimum."[44]

"Venus at times shows [the oxygen] green line emission with an intensity equal to terrestrial values [Slanger et al., 2001]. Furthermore, the intensity is quite variable, as is true for the much stronger O2( a-X) 1.27 μ emission."[45]

"In 1999, observations of the Venus nightglow with the Keck I telescope showed that the 5577 Å oxygen green line was a significant feature, comparable in intensity to the terrestrial green line. Subsequent measurements have been carried out at the Apache Point Observatory (APO) and again at Keck I, confirming the presence of the line with substantially varying intensity."[46]

In this International Space Station image, you can see green and yellow airglow paralleling the Earth’s horizon line (or limb) before it is overwhelmed by the light of the rising Sun. Credit: NASA Earth Observatory.

In the International Space Station image at right, you can "see green and yellow airglow paralleling the Earth’s horizon line (or limb) before it is overwhelmed by the light of the rising Sun. Airglow is the emission of light by atoms and molecules in the upper atmosphere after they are excited by ultraviolet radiation. ... Astronaut photograph ISS030-E-015491 was acquired on December 22, 2011, with a Nikon digital camera, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center."[48]

"As geomagnetic storms cause beautiful displays of aurora across the United States ..., astronauts onboard the International Space Station also have the opportunity to take a look. Credit: Earth Sciences and Image Analysis Laboratory, Johnson Space Center, NASA.

At right is a natural color photograph of the Aurora Borealis or northern lights and the Manicouagan Impact Crater reservoir (foreground) in Quebec, Canada. They are featured in this photograph taken by astronaut Donald R. Pettit, Expedition Six NASA ISS science officer, on board the International Space Station (ISS).

"Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented."[51]

"This is why there is a color differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly, pure blue."[51]

The third image is taken from the International Space Station. "Green colors of the aurora are dominant in this image captured by a digital still camera on October 4, 2001. Auroras are caused when high-energy electrons pour down from the Earth’s magnetosphere and collide with atoms. Green aurora occurs from about 100 km to 250 km altitude and is caused by the emission of 5577 Angstrom wavelength light from oxygen atoms. The light is emitted when the atoms return to their original unexcited state."[52]

Pale green when viewed through the water of the Persian Gulf, coral reefs fringe the shoreline and islands of the United Arab Emirates in this Landsat satellite image. Credit: Jesse Allen, NASA, United States Geological Survey.

"Pale green when viewed through the water of the Persian Gulf, coral reefs fringe the shoreline and islands of the United Arab Emirates in this Landsat satellite image. The reefs are more extensive than the islands they surround in many cases. The natural-color image was made from data collected by the Landsat 7 satellite on July 3 and July 10, 2002."[53]

"The reefs are an oasis of biological diversity, and they are among the most extensive reefs in the Gulf. Starting in January 2005, a division of the World Wildlife Federation in Abu Dhabi used Landsat images such as this one to create the first map of coral reefs off Abu Dhabi in the United Arab Emirates and Qatar. The map will help resource managers assess the quality and quantity of marine resources in the region and to make decisions that protect them."[53]

From a satellite's vantage point, the wash of green that appears across the forests of the eastern United States is one of the most noticeable signs of winter's passing. Credit: Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response and NASA's Earth Observatory.

"At ground level, tulips and daffodils mark the arrival of spring. But from a satellite's vantage point, the wash of green that appears across the forests of the eastern United States is one of the most noticeable signs of winter's passing. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Aqua satellite captured this view of spring greening on April 7, 2012, an unusually cloud free day."[54]

"The deepest greens of lush foliage are most visible throughout the Piedmont, a forested plateau between the Appalachians and the lower elevation plains along the Atlantic coast. The Appalachians appear brown because cooler temperatures at higher elevations cause a lag in the greening. In this case, the trees at the higher elevations were likely still in bloom and hadn't started to produce leaves. The speckles of tan throughout the coastal plain are farmlands, where fields often stay bare or filled with dry crop stubble until late spring planting."[54]

Pale green patterns tinted the water along the Namibian coast in late February 2012. Credit: Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC.

In the image at right, "[p]ale green patterns tinted the water along the Namibian coast in late February 2012. But unlike other bright hues that occasionally show up in the ocean, these colors didn’t result from a phytoplankton bloom.[55]

"[H]ydrogen sulfide gas is emitted periodically along the Namibian coast. Ocean currents carry oxygen-poor water to the region, and chemical and biological processes can deplete what little oxygen is available. The sediments in the local seafloor are also rich with organic matter. When organic matter decays in an oxygen-poor environment, hydrogen sulfide emissions can result."[55]

"The milky-green colors along Namibia’s coast indicate high concentrations of sulfur and low concentrations of oxygen. Episodes like this aren’t just colorful, they are actually toxic to local marine organisms. Fish die in the low-oxygen water; however, what is deadly for the fish can be good for birds that feed on their carcasses. Likewise, lobsters crawling onto shore to escape the toxic seawater can make meals for locals. And some species of foraminifera—tiny shelled marine organisms—actually thrive in the oxygen-poor sea floor sediments off the Namibian coast."[55]

"Phytoplankton swirled across the Arabian Sea on February 18, 2010, drawn into thin green ribbons by turbulent eddies. The bloom stretches from the shores of Pakistan (top) to the coast of Oman (lower left). The washed out appearance at the upper left of the image is due to sunglint, which is the mirror–like reflection of sunlight off the water. Some of the brightness may be caused by blowing dust. Pakistan’s coastal waters are tinged blue as well as green. The color may result from numerous influences, including phytoplankton and sediment."[56]

"The phytoplankton blooms in the northern Arabian Sea are strongly influenced by the seasonal wind shifts (the monsoon) that dominate the area’s climate. Because the Arabian Sea is landlocked to the north, it is less influenced by large-scale ocean circulation and more strongly influenced by the monsoon winds. Large blooms of phytoplankton occur in the summer, when strong southwesterly winds blow from the ocean toward land, mixing the water. Blooms also happen in the winter, however, when northeast winds blow offshore."[56]

As sediment disperses into the Gulf of Mexico, the water fades to green and then black. Credit: NASA image courtesy the MODIS Rapid Response Team at NASA GSFC.

"A burst of color lit the shallow waters of the Gulf of Mexico off the Yucatan Peninsula on December 14, 2008, when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this image [at right]. The swirls of tan, green, blue, and white are most likely sediment in the water. The sediment scatters light, giving the water its color. The sediment comes from two sources: the land and the sea floor. Some of the color may also come from phytoplankton, tiny plant-like organisms that live in the sun-lit surface waters of the ocean."[14]

"Near the shore, the water is tan where rivers carry dirt from land to the ocean. As the sediment disperses, the water fades to green and then black. To the north, the water is more blue and white than tan and green. In these regions, the sediment has likely come from the sea floor. Made up of chalky white calcium carbonate from shell-building marine life like coral, sea floor sediment gives the water a white or bright blue color. The sediment was probably brought to the surface in shallow waters by strong waves. A few days before the image was taken, strong winds churned the Gulf. The blue-green cloud in this image roughly matches the extent of the shallow continental shelf west of the peninsula."[14]

The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this natural-color image of the island and its offshore eruption on December 16, 2011. Credit: esse Allen and Robert Simmon, NASA EO-1 team, and NASA's Earth Observatory.

"After two months of eruptions and six months of quakes and tremors, the underwater volcano off the coast of El Hierro appears to be quieting down. A mass of new crust has been building by tens to perhaps a hundred meters, but it is still nowhere near rising above the ocean surface and extending the Canary Island chain."[57]

"The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this natural-color image of the island and its offshore eruption on December 16, 2011. The eruption is located about one kilometer south-southwest of the town of La Restinga."[57]

"The milky green swirls in the Atlantic Ocean are a volcanic brew of steaming lava fragments, bits of rock, heated gas, and other debris that are carried to the west and north by currents. Meanwhile, tremors have been occurring on the north side of the island at depths of 17 to 23 kilometers. Red circles mark the locations of three tremors reported on December 18. The white puffs over the island are clouds, not volcanic emissions."[57]

"El Hierro is a shield volcano growing along the southwestern edge of the ancient El Golfo volcano. El Golfo collapsed about 130,000 years ago, but the ruddy brown and rugged terrain of the island attests to many years of volcanism in the area. El Hierro last erupted in 1793, according to some historical records, and the area has the greatest concentration of young vents in the Canary Islands."[57]

This image shows weathered Precambrian pillow lava in the Temagami greenstone belt of the Canadian Shield in Eastern Canada. Credit: Black Tusk.

"Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies."[58]

"The name comes from the green hue imparted by the colour of the metamorphic minerals within the mafic rocks. Chlorite, actinolite and other green amphiboles are the typical green minerals."[58]

"A greenstone belt is typically several dozens to several thousand kilometres long and although composed of a great variety of individual rock units, is considered a 'stratigraphic grouping' in its own right, at least on continental scales."[58]

""Greenstone belts" are distributed throughout geological history from the Phanerozoic Franciscan belts of California where blueschist, whiteschist and greenschist facies are recognised, through to the Palaeozoic greenstone belts of the Lachlan Fold Belt, Eastern Australia, and a multitude of Proterozoic and Archaean examples."[58]

"Archaean greenstones are found in the Slave craton, northern Canada, Pilbara craton and Yilgarn Craton, Western Australia, Gawler Craton in South Australia, and in the Wyoming Craton in the US. Examples are found in South and Eastern Africa, namely the Kaapvaal craton and also in the cratonic core of Madagascar, as well as West Africa and Brazil, northern Scandinavia and the Kola Peninsula (see Baltic Shield)."[58]

"Phanerozoic ophiolite belts and greenstone belts occur in the Franciscan Complex of south-western North America, within the Lachlan Fold Belt, the Gympie Terrane of Eastern Australia, the ophiolite belts of Oman and around the Guiana Shield."[58]

"[T]he first identification of the OI 5577 Å green line [in a meteor spectrum was] (Halliday, 1959). ... the emission is a major contributor to wake spectra. ... emission occurs only from the upper part of the meteor light curve being generally confined to the height interval 100 - 110 km, and that its occurrence is rare in meteors whose velocities are less than about 40 kms-1."[59]

"Because of their difficult fusibility, extremely low water content, and its chemical composition, the current overwhelming consensus among Earth scientists is that moldavites were formed 15 million years ago during the impact of a giant meteorite in present-day Nördlinger Ries. Splatters of material that was melted by the impact cooled while they were actually airborne and most fell in central Bohemia—traversed by [the] Vltava river ... Currently, moldavites have been found in [an] area that includes southern Bohemia, western Moravia, the Cheb Basin (northwest Bohemia), Lusatia (Germany), and Waldviertel (Austria).[61]Isotope analysis of samples of moldavites have shown a beryllium-10 isotope composition similar to the composition of Australasian tektites (Australites)and Ivory Coast tektites (Ivorites). Their similarity in beryllium-10 isotope composition indicates that moldavites, Australites, and Ivorites consist of near surface and loosely consolidated terrestrial sediments melted by hypervelocity impacts.[62]"[60]

The Zodiacal Light is over the Faulkes Telescope, Haleakala, Maui. Credit: 808caver.

"The "Zodiacal light is a faint, roughly triangular, diffuse white glow seen in the night sky that appears to extend up from the vicinity of the Sun along the ecliptic or zodiac.[63] ... Caused by sunlight scattered by space dust in the zodiacal cloud, ... the zodiacal light covers the entire sky, being responsible for major part[64] of the total skylight on a moonless night."[65]

"[T]he distribution [is] over the sky of the linearly polarized component of the zodiacal light at λλ5080 Å and 5300 Å. In the antisolar hemisphere the distribution shows three clear effects: (i) There is a slight (~ 3°) clockwise overall rotation of the orientation of the polarization plane relative to that calculated for spherical interplanetary dust; (ii) near the antisolar point the polarization plane tends to be normal to the ecliptic, but the distribution of the polarization vectors is asymmetric with respect to the ecliptic plane ("north-south asymmetry"); (iii) significant day-to-day and month-to-month changes occur in the zodiacal light but without clear trends."[66]

This visual image of the Moon shows sunlight reflected as the bright crescent and earthlight reflected from the dark side. Credit: Ilmari Karonen.

Def. "sunlight reflected from the earth's surface"[67] is called earthlight.

Def. "[r]eflected earthlight visible on the Moon's night side"[68] is called earthshine.

"The search for life on extrasolar planets" requires a test of vegetation detectability from a single dot source.[69]

"The earthshine, or ashen light, is the glow of the dark part of the lunar disk visible to a night-time observer. ... [T]he light rays coming from different parts of the Earth are mixed together in the ashen light and mimic the Earth as a single dot."[69]

"[T]he vegetation spectrum which is unequivocal ... presents a bump at 0.5 µ in the green wavelength range, which implies that plants appear green".[69]

This colour mosaic was assembled from 18 images taken by Galileo's imaging system through a green filter. Credit: NASA/JPL/USGS.

"During its mission, the Galileo spacecraft returned a number of images of Earth's only natural satellite. Galileo surveyed the moon on Dec. 7, 1992, on its way to explore the Jupiter system in 1995-1997."[70]

"This color mosaic was assembled from 18 images taken by Galileo's imaging system through a green filter. On the upperleft is the dark, lava-filled Mare Imbrium, Mare Serenitatis (middle left), Mare Tranquillitatis (lower left), and Mare Crisium, the dark circular feature toward the bottom of the mosaic. Also visible in this view are the dark lava plains of the Marginis and Smythii Basins at the lower right. The Humboldtianum Basin, a 400-mile impact structure partly filled with dark volcanic deposits, is seen at the center of the image."[70]

This is the first image of the asteroid Ida using the green 559 nm filter onboard the Galileo spacecraft. Credit: NASA/JPL.

"This is the first full picture showing both asteroid 243 Ida and its newly discovered moon to be transmitted to Earth from the National Aeronautics and Space Administration's (NASA's) Galileo spacecraft--the first conclusive evidence that natural satellites of asteroids exist. Ida, the large object, is about 56 kilometers (35 miles) long. Ida's natural satellite is the small object to the right. This portrait was taken by Galileo's charge-coupled device (CCD) camera on August 28, 1993, about 14 minutes before the Jupiter-bound spacecraft's closest approach to the asteroid, from a range of 10,870 kilometers (6,755 miles). Ida is a heavily cratered, irregularly shaped asteroid in the main asteroid belt between Mars and Jupiter--the 243rd asteroid to be discovered since the first was found at the beginning of the 19th century. Ida is a member of a group of asteroids called the Koronis family. The small satellite, which is about 1.5 kilometers (1 mile) across in this view, has yet to be given a name by astronomers. It has been provisionally designated '1993 (243) 1' by the International Astronomical Union. ('1993' denotes the year the picture was taken, '243' the asteroid number and '1' the fact that it is the first moon of Ida to be found.) Although appearing to be 'next' to Ida, the satellite is actually in the foreground, slightly closer to the spacecraft than Ida is. Combining this image with data from Galileo's near-infrared mapping spectrometer, the science team estimates that the satellite is about 100 kilometers (60 miles) away from the center of Ida. This image, which was taken through a green filter, is one of a six-frame series using different color filters. The spatial resolution in this image is about 100 meters (330 feet) per pixel."[73]

Gases above Io's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). Credit: NASA/JPL/University of Arizona.

At right is an "eerie view of Jupiter's moon Io in eclipse ... acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The vivid colors, caused by collisions between Io's atmospheric gases and energetic charged particles trapped in Jupiter's magnetic field, had not previously been observed. The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter."[74]

"North is to the top of the picture, and Jupiter is towards the right. The resolution is 13.5 kilometers (8 miles) per picture element. The images were taken on May 31, 1998 at a range of 1.3 million kilometers (800,000 miles) by Galileo's onboard solid state imaging camera system during the spacecraft's 15th orbit of Jupiter."[74]

The image at the left was taken in visible green light. The image at the right has been color-coded to make faint signals in the plume more apparent. Credit: NASA.

At right is a pair of images showing "[a] fine spray of small, icy particles emanating from the warm, geologically unique province surrounding the south pole of Saturn’s moon Enceladus[. It] was observed in a Cassini narrow-angle camera image of the crescent moon taken on Jan. 16, 2005. Taken from a high-phase angle of 148 degrees -- a viewing geometry in which small particles become much easier to see -- the plume of material becomes more apparent in images processed to enhance faint signals [right image of the pair]."[75]

"Though the measurements of particle abundance are more certain within 100 kilometers (60 miles) of the surface, the values measured there are roughly consistent with the abundance of water ice particles measured by other Cassini instruments (reported in September, 2005) at altitudes as high as 400 kilometers (250 miles) above the surface."[75]

"The image at the left was taken in visible green light. A dark mask was applied to the moon's bright limb in order to make the plume feature easier to see."[75]

"The image at the right has been color-coded to make faint signals in the plume more apparent. Images of other satellites (such as Tethys and Mimas) taken in the last 10 months from similar lighting and viewing geometries, and with identical camera parameters as this one, were closely examined to demonstrate that the plume towering above Enceladus' south pole is real and not a camera artifact. The images were acquired at a distance of about 209,400 kilometers (130,100 miles) from Enceladus. Image scale is about 1 kilometer (0.6 mile) per pixel."[75]

This is an image of Iapetus in polarized green light with the Cassini spacecraft narrow-angle camera. Credit: NASA/JPL/Space Science Institute.

The color seen here on this false-color composite of Iapetus is similar to that produced in (red, green and blue) natural color views. Credit: NASA / JPL / Space Science Institute.

"Although it is no longer uncharted land, the origin of the dark territory of Cassini Regio on Iapetus remains a mystery. Also puzzling is the equatorial ridge that bisects this terrain, and how it fits into the story of the moon's strange brightness dichotomy. The ridge is seen here, curving along the lower left edge of Iapetus. The view looks down onto the northern hemisphere of Iapetus (1,468 kilometers, or 912 miles across), and shows terrain on the moon's leading hemisphere."[76]

"The image was taken in polarized green light with the Cassini spacecraft narrow-angle camera on Nov. 12, 2005 at a distance of approximately 417,000 kilometers (259,000 miles) from Iapetus and at a Sun-Iapetus-spacecraft, or phase, angle of 95 degrees. Image scale is about 2 kilometers (1 mile) per pixel."[76]

In the second image are three "different false-color views of Saturn's moon Iapetus show the boundary of the global "color dichotomy" on the hemisphere of this moon facing away from Saturn. The "color dichotomy," which has been detected in images from the Cassini imaging team, is a second global pattern found on Iapetus besides the well-known global brightness dichotomy."[77]

"This image consists of three panels, each of which was contrast-enhanced in different ways to bring out surface features. Minimal enhancement was applied to the image on the left panel while those on the middle and right panels were enhanced more (with contrast increased by factors of two and four, respectively), making them appear brighter and overexposed."[77]

"In the case of the color dichotomy seen here, its boundary is quite well correlated with the boundary between the moon's leading and the trailing hemispheres. At near-infrared wavelengths, the bright terrain on the leading side is redder than on the trailing side. This pattern is visible in the panel on the left, which uses normal contrast enhancement. The characteristic reddish distribution also appears on the dark material, as seen in the middle and right-hand panels that have been adjusted with even higher contrast. Indeed, the otherwise uniformly dark material shows different color hues, depending on whether the viewer looks at the leading vs. the trailing side."[77]

This image of Tethys is taken in visible green light with the Cassini spacecraft narrow-angle camera on September 14, 2011. Credit: NASA Cassini.

At right is an image of Saturn's moon Tethys in green light.

"On the top left of the image there is huge Odysseus Crater. See PIA07693 for a closer view. On the bottom right there is Ithaca Chasma, a series of scarps that runs north-south across the moon for more than 620 miles (1,000 kilometers). North on Tethys is up and rotated 25 degrees to the right."[78]

"This view looks toward the area between the leading hemisphere and Saturn-facing side of Tethys (660 miles, or 1,062 kilometers across)."[78]

"The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Sept. 14, 2011. The view was acquired at a distance of approximately 178,000 miles kilometers (287,000) from Tethys and at a Sun-Tethys-spacecraft, or phase, angle of 11 degrees. Image scale is about 1 mile (2 kilometers) per pixel."[78]

This is a visual image of Uranus in the green. Credit: Heidi Hammel (Massachusetts Institute of Technology), NASA.

"Spring has finally come to the northern hemisphere of Uranus. The newest images, both the visible-wavelength ones described here and those taken a few days earlier with the Near Infrared and Multi-Object Spectrometer (NICMOS) by Erich Karkoschka (University of Arizona), show a planet with banded structure and detectable clouds."[79]

"The "aqua" image (on the right) is taken at 5,470 Angstroms, which is near the human eye's peak response to wavelength. Color has been added to the image to show what a person on a spacecraft near Uranus might see. Little structure is evident at this wavelength, though with image-processing techniques, a small cloud can be seen near the planet's northern limb (rightmost edge)."[79]

This is a 3D model of the Rosetta Spacecraft. The individual scientific payloads are highlighted in different colours. Credit: IanShazell.

For elongated dust particles in cometary comas an investigation is performed at 535.0 nm (green) and 627.4 nm (red) peak transmission wavelengths of the Rosetta spacecraft's OSIRIS Wide Angle Camera broadband green and red filters, respectively.[80] "In the green, the polarization of the pure silicate composition qualitatively appears a better fit to the shape of the observed polarization curves".[80] "[B]ut they are characterized by a high albedo."[80] The silicates used to model the cometary coma dust are olivene (Mg-rich is green) and the pyroxene, enstatite.[80]

"[U]nequivocal detections [occurred at McDonald Observatory on 10 nights from 25 June through 17 July 2000] of the O (1S) and O (1D) metastable lines in emission in the cometary [Comet C/1999 S4 (LINEAR)] spectrum. These lines are well separated from any telluric or cometary emission features."[81]

"[T]he presence of the [oxygen] green line can still be questioned, unless the 2972 Å trans-auroral line [1S - 3P] is detected (Herbig, 1976)."[36] "The transitions involved (allowed and forbidden) in the spectrum of the oxygen atoms in a cometary atmosphere" are 557.7 nm, 630.0 and 636.4 nm, 295.8 and 297.2 nm, 98.9 nm (a triplet), 799.0 nm, 844.7 nm, and 1304 nm (a triplet), 102.7 nm (a triplet) and 1128.7 nm.[36]

"When the green line is overwhelming (in faint comets like Encke), this emission is mainly due to the airglow, the red airglow emission being quenched and consequently weaker than the green."[36]

"The measured intensity on 10 January 1980, when the comet was 0.71 a.u. from the Sun and 0.615 a.u. from the Earth, is 30±15 Rayleighs.[36]

"In December 2006, seven papers were published in the scientific journal, Science, discussing initial details of the sample analysis. Among the findings are: a wide range of organic compounds, including two that contain biologically usable nitrogen; indigenous aliphatic hydrocarbons with longer chain lengths than those observed in the diffuse interstellar medium; abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene, proving consistency with the mixing of solar system and interstellar matter, previously deduced spectroscopically from ground observations;[82] hydrous silicates and carbonate minerals were found to be absent, suggesting a lack of aqueous processing of the cometary dust; limited pure carbon (CHON) was also found in the samples returned; methylamine and ethylamine was found in the aerogel but was not associated with specific particles."[83]

This is an image of NGC 2080, the Ghost Head Nebula. Credit: NASA, ESA and Mohammad Heydari-Malayeri (Observatoire de Paris, France).

At right is a Hubble Space Telescope image of the Ghost Head Nebula. "This nebula is one of a chain of star-forming regions lying south of the 30 Doradus nebula in the Large Magellanic Cloud. The red and blue light comes from regions of hydrogen gas heated by nearby stars. The green light comes from glowing oxygen, illuminated by the energy of a stellar wind. The white center shows a core of hot, massive stars."[84]

This is a wide-field view of the sky around the bright G2V star Alpha Centauri A, created from photographic images forming part of the Digitized Sky Survey 2. Credit: ESO/Digitized Sky Survey 2.

This is an optical image of Capella B. Credit: Aladin at SIMBAD.

"Alpha Centauri A is the principal member, or primary, of the binary system, being slightly larger and more luminous [151.9% the luminosity of the Sun] than the Sun. It is a solar-like main sequence star with a similar yellowish color,[85] [surface temperature of 5790 K][86] whose stellar classification is spectral type G2 V.[87] From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than the Sun, with a radius about 23% larger.[86] The projected rotational velocity( v·sin i ) of this star is 2.7 ± 0.7 km·s−1, resulting in an estimated rotational period of 22 days,[88] which gives it a slightly faster rotational period than the Sun's 25 days."[89]

Capella B, the second image at right, "has a surface temperature of approximately 5700 K, a radius of approximately 9 solar radii, a mass of approximately 2.6 solar masses, and a luminosity, again measured over all wavelengths, approximately 78 times that of the Sun.[90]"[91]

Capella B is spectral type G0III star and an X-ray source from the catalog [FS2003] 0255 by ROSAT. Its surface temperature has an uncertainty of 100 K. It is part of a binary star with Capella A a G8III. From SIMBAD, the orbital period is 104.0217 d with an eccentricity is 0.001 and inclination of 137.2° to the line of sight.

Although the binary star Capella is not an eclipsing binary, it is a RS Canum Venaticorum variable. These "are close binary stars[92] having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity. Systems can exhibit variations on timescales of years due to variation in the spot surface coverage fraction, as well as periodic variations which are, in general, close to the orbital period of the binary system. ... Typical brightness fluctuation is around 0.2 magnitudes."[93]

This is a Hubble Space Telescope image of the green continuum in the nuclear region of NGC 5728. The observation was made on September 4, 1992, using the Planetary Camera with appropriate filters. Credit: A. S. Wilson, J. A. Braatz, T. M. Heckman, J. H. Krolik, & G. K. Miley/NASA-Hubble.

This is a Hubble Space Telescope image of the green [O III] emission line in the active galactic nuclear region of NGC 5728. The observation was made on September 4, 1992, using the Planetary Camera with appropriate filters. Credit: A. S. Wilson, J. A. Braatz, T. M. Heckman, J. H. Krolik, & G. K. Miley/NASA-Hubble.

"[H]igh-resolution (0.1") observations of the Seyfert 2 galaxy NGC 5728 with the Hubble Space Telescope",[16] in the images at right, show the full color of the galactic nucleus and a green continuum image of the active galactic nucleus.

At left is a Hubble Space Telescope image in the light of the green [O III] emission line in the active galactic nuclear region of NGC 5728. "The emission-line images reveal a spectacular biconical structure with overall extent [of] 1.8 kpc. The two cones share a common axis and apex. The cones' axis coincides to within ≃ 3° with that of the one-sided nuclear radio continuum emission but is not aligned with the rotation axis of the galaxy disk."[16]

These "ionization cones" are "conical regions of high-excitation emission-line gas extending from an active nucleus. ... The generally accepted interpretation is that partially collimated ionizing radiation shines out from the nucleus and ionizes gas in its vicinity. ... two exposures were taken through the F492M filter (4906 Å/364 Å) to cover the [O III] λλ4959, 5007 emission. ... The high-excitation gas can be traced to ≃ 8.5" (1.6 kpc) from the apex in the SE cone, but only to 1.5" (270 pc) in the NW one."[16]

"Osiris is the mythological father of the god Horus, whose conception is described in the Osiris myth, a central myth in ancient Egyptian belief. The myth described Osiris as having been killed by his brother Seth, who wanted Osiris' throne. Isis joined the fragmented pieces of Osiris, but the only body part missing was the phallus. Isis fashioned a golden phallus, and briefly brought Osiris back to life by use of a spell that she learned from her father. This spell gave her time to become pregnant by Osiris before he again died. Isis later gave birth to Horus. As such, since Horus was born after Osiris' resurrection, Horus became thought of as a representation of new beginnings and the vanquisher of the evil Set."[94]

"The Phoenician El - Saturn - has four eyes, as does the Orphic Kronos (Saturn)."[95]

"The Chinese Yellow Emperor Huang-ti--identified as Saturn--is also four-eyed.74"[95]

"Osiris, as the Ram of Mendes, is the god of "four faces on one neck."62"[95]

c2 = 1.438833 cm K may be used to approximate a pair such as (495 nm, 6100 K).

For a simple calculator, y=(1.48833/(0.0000570*5260))*exp(1.48833/(0.0000570*5260))/(exp(1.48833/(0.0000570*5260))-1)-5, followed by print y, yields a value close to zero (-1.006663E-03). The closer to zero the more accurate the estimate.

Although Planck's equation is not an exact fit to a star's spectral radiance, it may be close enough to suggest if a star is an astronomical green source using the above derivative.

From a Planckian spectrum peaked in the green radiation band the wavelength temperature pairs are approximately (495 nm, 6100 K) and (570 nm, 5260 K).

As the Sun has a surface temperature of 5777 K, it is a green radiation source as a Planckian radiator.

The Hubble Space Telescope is seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final human spaceflight to visit the observatory. Credit: Ruffnax (Crew of STS-125).

The HST is an optical astronomy telescope that “incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest."[96]

The Wide Field Planetary Camera (PC-1) was in use from about 1990 through 1993. It carried 48 filters on 12 filter wheels of four each. For the green band, these were the F492M, F502N, F517N, F547M, and the F555W. Those ending in 'N' are narrow-band filters.[97]

One of these filters is F492M which allows imaging with the [O III]λλ4959,5007 and its adjacent green continuum. The filter band pass is centered at 490.6 nm with a full-width at half maximum (FWHM) of 36.4 nm.[16] "The F492M filter also includes Hβ.[16]

The F502N is centered at 501.85 nm with a band pass of 2.97 nm. The F547M is centered at 546.1 nm with a band pass of 43.8 nm.

The Wide Field Planetary Camera (PC-2) replaced PC-1 and carried the following filters on the same filter wheels: F467M, F502N, F547M, F555W, and the F569W.[97] In December 1993 PC-1 was replaced with PC-2 and the HST was declared operational on January 13, 1994.[96]

Onboard the HST is the Faint Object Camera (FOC) which carries filters for green astronomy: F470M, F480LP, F501N, F502N, and the F550M.[97]

This image shows the three main spectral lines that Narrowband filters transmit. Credit: Vedran V.

"Most astronomical filters work by blocking a specific part of the color spectrum above and below a bandpass, significantly increasing the signal to noise of the interesting wavelengths, and so making the object more visible, 'contrasty', or defined. ... The broadband and narrowband filters transmit the wavelengths that are emitted by the nebulae (by the Hydrogen and Oxygen atoms), and are frequently used for reducing light pollution.[99]"[100]

"Color filters work by absorption/transmission, and can tell which part of the spectrum they are reflecting and transmitting. Filters can be used to increase contrast and enhance the details of the Moon and planets. All of the visible spectrum colors each have a filter, and every color filter is used to bring a certain lunar and planetary feature; for example, the #8 yellow filter is used to show Mars's maria and Jupiter's belts.[101] The Wratten system is the standard number system used to refer to the color filter types. ... Professional filters are also colored, but their bandpass centers are placed around other mid-points (such as in the UBVRI and Cousins systems)."[100]

The "Dark green: Improves cloud patterns on Venus. Reduces sky brightness during daylight observation of Venus. Increases contrast of ice and polar caps on Mars. Improves visibility of the Great Red Spot on Jupiter and other features in Jupiter atmosphere. Enhances white clouds and polar regions on Saturn."[100]

"Narrowband filters are astronomical filters which transmit only a narrow band of spectral lines from the spectrum (usually 22 nm or less). It is mainly used for nebulae observation. Emission nebulae mainly radiate the doubly ionized oxygen in the visible spectrum, which emits near 500 nm wavelength. These nebulae also radiate weaker at 486 nm from the Hydrogen-beta atoms. There are three main types of Narrowband filters: Ultra-high contrast (UHC), Oxygen-III & Hydrogen-beta, and Hydrogen-alpha, the narrowest of the three filters with 8 nm range. The UHC filters range from 484 to 506 nm.[101] It transmits both the O-III and H-beta spectral lines, blocks a large fraction of light pollution, and brings the details of planetary nebulae and most of emission nebulae under a dark sky.[102]"[100]

"The Broadband or light pollution reduction (LPR) filters are nebular filters that block the light pollution in the sky and transmit the H-alpha, H-beta, and O-III spectral lines, which makes observing nebulae from the city and light polluted skies possible.[99] These filters block the Sodium and Mercury vapor light, and also block the natural skyglow such as the auroral light.[103] The broadband filters differ from the narrowband with the range of wavelengths transmission. The broadband filters have a wider range because the narrower transmission range causes a fainter image of sky objects, and since the work of these filters is revealing the details of nebulae from light polluted skies, it has a wider transmission for more brightness.[101] These filters are particularly designed for nebulae observing, are not useful with other deep sky objects. However, it can improve the contrast between the DSOs and the background sky, which may clarify the image."[100]

The findings demonstrate a statistically systematic change from the status quo or the control group.

“In the design of experiments, treatments [or special properties or characteristics] are applied to [or observed in] experimental units in the treatment group(s).[105] In comparative experiments, members of the complementary group, the control group, receive either no treatment or a standard treatment.[106]"[107]

↑ 5.05.1(Oxford English Dictionary, 2nd Edition, Clarendon Press, Oxford, 1989.) See also first definition in Webster's New World Dictionary of the American Language, The World Publishing Company, New York, 1962.